The Obama administration on Monday granted new powers to federal regulators fighting gun traffickers on the violence-plagued Mexican border.

Issued by the Department of Justice (DOJ), the new rules require border-state gun dealers to report bulk purchases of assault weapons made by individual buyers over short spans of time — a tool requested in December by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF).

Supporters say the new oversight will help reduce the flow of guns to the violent drug cartels south of the border.

“The international expansion and increased violence of transnational criminal networks pose a significant threat to the United States,” Deputy Attorney General James Cole said in a statement. The new powers, he added, will help ATF officials “disrupt the illegal weapons trafficking networks responsible for diverting firearms from lawful commerce to criminals and criminal organizations.”

The change drew the immediate praise of gun reformers, including Rep. Elijah Cummings (D-Md.), who said it represents “a crucial tool to identify and disrupt Mexican drug cartels.”

But Rep. Lamar Smith (R-Texas), chairman of the House Judiciary Committee, slammed the change as an erosion of constitutional rights.

“Limiting the Second Amendment rights of law abiding citizens is not going to solve the problem of guns being trafficked into Mexico,” Smith said in a statement. “This rule unfairly punishes citizens in border states who have the right to purchase firearms to protect themselves and their families from dangerous drug traffickers and human smugglers.”

Smith was quick to hammer the administration for a controversial ATF program — dubbed Fast and Furious — designed to track firearms by letting known smugglers buy them with impunity. Several of those guns have been linked to the murder of Border Patrol Agent Brian Terry, who was killed in a December firefight in Arizona.

“It is the height of hypocrisy for the Obama administration to restrict the gun rights of border state citizens, when the administration itself knowingly and intentionally allowed guns to be trafficked into Mexico,” Smith said.

Democrats, including Cummings, have been critical of the Fast and Furious program. But they also view the botched sting operation as a sign that federal regulators simply lack the tools to police the border within the confines of current law.

The Maryland Democrat said it’s Congress’s responsibility to ensure that ATF regulators “are not outgunned” at the border.

Along with Rep. Carolyn Maloney (D-N.Y.), Cummings is drafting legislation to encourage gun prosecutors to go after gun traffickers by attaching stiffer penalties to crime. The legislation is expected to be released in the coming weeks.

Mexican drug violence has killed more than 30,000 people since 2006, when Mexican President Felipe Calderón launched a crackdown against the country’s powerful cartels. Thousands of guns used by the smugglers have originated with gun dealers in the United States, according to numerous reports from the ATF and gun-control advocates.

The new rules require licensed gun dealers in the border states to report sales of two or more assault weapons to the same buyer within five work days. A similar mandate has applied to handgun purchases for more than 40 years — something that hasn’t been overlooked by gun-reform advocates.

“For many years, dealers have been required to report multiple sales of handguns, and the program has been a critical tool for the ATF in attempting to break up gun-trafficking rings,” New York Mayor Michael Bloomberg, co-chairman of Mayors Against Illegal Guns, said in a statement. Expanding the mandate “does nothing to interfere with the rights of law-abiding Americans,” Bloomberg added.

“It’s a logical step that will help law enforcement agents fight Mexican gun runners.”

The requirement applies only to dealers in Arizona, California, New Mexico and Texas. Affected guns must be semi-automatic, must carry ammunition larger than .22-caliber, and must feature a detachable ammunition clip — characteristics favored by the Mexican drug cartels.

Citing the severity of the Mexican gun violence, the ATF in December asked the White House to fast-track the new reporting requirement — a request the administration denied.

The issue has already been broached by Congress. In February, as part of the GOP’s proposal to cut $61 billion in federal spending this year, House Republicans passed legislation blocking the ATF’s reporting requirement. That bill was killed in the Senate, and Democrats stripped the provision from the final 2011 spending bill.